Means for safeguarding electric igniters of blasting detonators against accidental firing



Sept. 24, 1946 H. J. ROLFES MEANS FOR SAFEGUARDING ELECTRIC IGNITERS OF BLASTING DETONATORS AGAINST ACCIDENTAL FIRING. Filed Aug. 18, 1942 2 Sheets-Sheet l p 946. H. J. ROLFES MEANS FOR SAFEGUARDING ELECTRIC IGNITERS OF BLASTING 'DETONATORS AGAINST ACCIDENTAL FIRING Filed Aug. 18, 1942 2 Sheets-Sheet 2 Fi5 F5 Patented Sept. 24, 1946 also STATES Hans Jay Rolfes, Melrose, Johannesburg, Transvaal, Union of South Africa Application August 18, 1942, Serial No. 455,235. In the Union of South Africa September 11,

4 Claims. I

This invention relates to means for safeguarding the electric igniters of blasting detonators against accidental firing by electric currents or spark discharges produced respectively in, or in the vicinity of, the firing means of the igniter by atmospheric electricity.

It is obvious that a possible solution to this problem is to surround the two electric leads of the igniter at a position a safe distance from the firing means thereof by a grounded mass of a material, which may be termed the ideal material, and which will exhibit a high resistance at voltages of the magnitude used for intentionally firing the igniter and a low resistance at voltages of the magnitude of the electrostatic charges which produce the currents and spark discharges aforesaid.

One means of obtaining a solution of the problem has already been proposed, the means consisting broadly in employing for the purposes of the material surrounding the leads any material which has the characteristic that its resistance to electric currents or discharges is an inverse function of the voltage applied thereto. Amongst materials having this property are, for example, galena, stibnite, carborundum, yellow crystalline iron pyrites and zincite.

This application is a continuation in part of my copending application No. 422,452.

The object of the present invention is to provide another means of solving the problem stated which will render available as the ideal material a class of materials other than those of which the resistance is an inverse function of the applied voltage as aforesaid, and which will afford a particularly efiective safeguard against currents or spark discharges produced in, or in the vicinity of the firing means of the igniter by electrostatic charges caused by lightning or atmospheric electricity in general.

According to this invention, means for safeguarding electric igniters of blasting detonators against accidental firing by electric currents or spark discharges produced respectively in, or in the vicinity of, the firing means of the igniter by atmospheric electricity, comprises a grounded mass of a material which offers a complete or substantially complete resistance to voltages of the magnitude used for intentionally firing the igniter and which, on its being subjected to voltages of the magnitude of electrostatic charges caused by atmospheric electricity, becomes locally conductive through coherer action along a path or paths permitting the passage therethrough of the high voltage charge.

More specifically, the invention consists in means for safeguarding as aforesaid electric igniters which includes a plug of material which is grounded through the shell of the igniter or detonator or otherwise and which comprises minute metal or other electrically conductive particles which in the mass are non-conductive to voltages of the magnitude used for intentionally firing the detonator and which in the mass become instantly conductive through coherer action to currents or electric discharges at voltages of the magnitude resulting from atmospheric electricity along a locally confined path from the lead or leads to ground.

The grounded mass may surround the electric leads, e. g. igniter wires or wires of the blasting circuit, at parts located inside and/or outside the igniter, these parts being bared; or the igniter wires or blasting circuit wires may be tapped by insulated conductors having bared parts surrounded by the grounded mass.

The invention will now be described by way of example with reference to the annexed drawings, in which:

Fig. 1 is a sectional view in elevation of an instantaneous electric detonator embodying the invention.

Fig. 2 is a sectional view in elevation of an instantaneous electric detonator, enclosed in an inert body or so-called wax primer, embodying the invention.

Figs. 3 and 4 are sectional views in elevation of electric igniters for delay-action detonators, embodying the invention.

Fig. 5 is a diagrammatic view of an alternative means for associating the igniter leads indirectly with each other through a coherer mass.

Figs. 6 and '7 are diagrammatic views of two embodiments of this alternative means applied to groups of detonators connected in series.

Figs. 8 and 9 are respectively a sectional elevation, and a sectional plan, of a convenient practical form of the alternative means.

Figs. 10 and 11 are respectively a sectional elevation and sectional plan of an improvement in the alternative means.

Referring to Figs. 1 to 4:

In Fig. 1, the insulated electric leads I I pass through a rubber plug 2 crimped in the mouth of the copper or other conductive shell 4 of the detonator. In the usual manner the detonator shell 4 contains a fuze-head 5, a primary charge 6 and secondary charge I. The fuze-head 5 has a bridge-wire 8 within the igniter composition or bead thereof, and the one end of the bridge-wire 3 is connected to a bridge-pole 9 and the other end to a bridge-pole I3. These poles 9, H! are connected at soldering points M, 2 to the respective leads I l Parts i3, M of theleads l l respectively are bared, as shown in the drawings, and these parts pass in spaced relationship through a plug I 5 provided within the detonator shell 4 between the rubber plug 2 and the insulating sleeve or body 55 of the fuze-head 5.

The plug l5, hereinafter referred to as the coherer plug, consists of finely divided aluminium particles which in the mass are non-conductive to Voltages of the magnitude used for intentionally firing the detonator and which in the mass become conductive through coherer action to currents or electric discharges at voltages of the magnitude resulting from atmospheric electricity.

It has been discovered as a result of experiment that finely divided aluminium used in the paint trade is a particularly suitable material for the coherer plug. Such aluminium appears to be in the form of minute flakes which have a film of grease or oxide over them, which may account for their exceptional non-conductivity in the mass to normal voltages. Preferably the aluminium powder is mixed with oil to form a paste, or with liquefied paraffin wax to form a melt or heat plastic mass, so that the requisite amount may be squirted as a plug into the igniter shell. The aluminium powder may, on the other hand, be mixed with rubber, or rubber-like or other plastic materials, and, in any case, be used to replace the rubber plug or sealing means at the mouth of a detonator, or alternatively it may be used in place of the insulation mass commonly employed for the leads or for part of the wires in the blastingcircuit. .Other substances displaying coherer action are finely powdered silver, nickel, iron, brass and cop-per and these substances can also be used for the coherer plug, being preferably mixed for this purpose with paraffin wax or-other fusible, powdered or liquid dielectric. Another material which has been found suitable for the coherer plug is graphite, preferably in defiocculated state, mixed with a viscous neutral hydrocarbon such as light lubricating oil to form a paste. Through varying the amount of oil admixed with the graphite, and by adding other inert materials which may influence the physical properties of the paste, it is possible to vary to a large extent the resistance to the firing current without impairing the coherer action with respect to the high voltage" discharges.

Teeth, projections, edges or other means may be formed or provided on the igniter leads at their bared parts surrounded by the coherer mass, and/or on the interior of the casing containing the said mass, or otherwise so as to constitute discharge points which promote the discharge of the high voltage charge from the leads or either of them into the said mass, as described in my co-pending patent application No. 422,452. The bared parts l3, I4 of the leads l 1' are formed, in the particular example shown in Fig. 1, with pointed teeth ll, l8 respectively directed diametrically outwards towards the wall of the detonator shell 5. Any projections, edges or other means adapted to constitute the discharge points may be employed in lieu of the particular teeth [1, I8 illustrated, so long as they fulfill the purpose aforesaid of promotin discharge of a high voltage charge from the leads I l or either of them, into the coherer plug l5.

When the detonator is fired, the firing current passes in the normal manner through the one lead, say l across the bridge-wire S, and out through the other lead I, the bridge-wire bein heated to incandescence and firing the igniter bead of the fuze-head 5. In other words, to the voltage of the magnitude used for the firing current, the coherer plug I5 reacts as an insulator. If, on the other hand, an electrostatic charge of high potential is induced in the firing circuit consisting of the leads l l and bridge-wire 8, or if this circuit is struck by a lightning discharge, the finely divided aluminium of the coherer plug I5 becomes at once locally conductive through coherer action along outward radial paths from the teeth I1, E8 to the wall of the detonator shell 4, permitting the passage through these paths of the high Voltage charge from the firing circuit to ground. In this manner the firing circuit discharges its high potential to ground without any flow of current being produced in the bridge-wire 8, or spark discharges being produced in the vicinity of the fu'ze-head 5, such as would cause incandescence of the bridge-wire or premature ignition of the fuze-head.

By using the teeth IE, it or similar discharge points, an effective safeguard is provided against the consequences of a high voltage electric surge which may arise in one lead only of the detonator. The discharge teeth (say the teeth I! if the surge arises in the lead'l) elfect complete discharge of that surge direct to the detonator shell 4, without any residual surge passing on to the bridge-wire 8. In this case also, therefore, there is no flow of current through the bridgewire such as might cause incandescence and premature firing of the fuze-head 5.

The coherer mass may be used not only in the form of a plug enclosed within the igniter or detonator shell, but also in the form of a plug or layer surrounding the leads at a place outside the igniter or detonator. Fig. 2 shows an application of this to a detonator enclosed in an inert body or so-called wax primerZO. Reference numerals used in this figure similar to those used in Fig. 1 refer to similar parts. In this embodiment, the coherer mass 2| is made in the form of a layer interposed between the inert or wax body 20 and a top inert or wax layer 22, The leads l l are bared and formed with outwardly directed discharge teeth at 23 and 24 respectively, where they pass through the coherer layer 2|.

In function, the embodiment shown in Fig. 2 is similar to that already described with reference to Fig. 1, and, as shown in Fig. 2, the toothed and bared parts 23, 24 of the leads I I are preferably spaced outwardly from each other in the coherer layer 2| so as to afford high voltage discharges a 'readier path to ground, 1. e. to the rock or side of the borehole in which the wax primer is inserted.

In Fig. 3, similar numerals again denote similar parts. The embodiment shown in this figure is an electric igniter having a copper or other conductive cylindrical shell 30 into the lower open end of which there is crimped a flexible delay fuze 3!, of which the other end (not shown) is crimped in known manner in the mouth of a detonator. A coherer plug 32 is in' this instance inserted in the igniter shell 30 so as to surround the solder points H, 12, the adjacent parts of the bridge-poles 9, l0, and the bared lower ends 33, 34 of the leads l l The upper end of the '5 igniter shell 30 is closed by a rubber plug 35 crimped therein.

Fig. 4 Shows the application of the invention to a delay-action detonator of the Eschbach type.

The construction of this detonator is similar to that of the instantaneous detonator shown in Fig. 1, with the exception that, between the fuzehead 5 and the primary charge 6, a delay composition 3 i (e. g. antimony and potassium permanganate) contained in a lead body 3! is provided. Otherwise, similar reference numerals in the two figures denote similar parts.

In any of the embodiments of the invention, more than one grounded coherer plug or mass may be used in association with the one detonator or igniter. One form of this modification is shown incorporated in the embodiment illustrated in Fig. 3 where, in addition to the coherer plug 32 surrounding the bared parts 33, 34 of the leads within the igniter shell 30, a second coherer plug 35 is contained in a separate copper or other conductive sleeve or shell 31 which is crimped on rubber plugs 38, 33 as an independent unit around bared parts 48, 4| of the leads l l outside the igniter shell 38.

When more than one grounded coherer plu or mass is thus used, discharge of high voltage charges from the firing circuit to ground may take place through either one or other or both of the said plugs or masses. In carrying out the invention. steps may, if necessary, be taken to ensure that high voltage discharges should they occur, will not take place through the coherer mass from one lead of the igniter or detonator across to the other. A safeguard against this happening is the provision. of discharge points hereinbefore referred to, Should these points not be employed, as for example in the embodiment illustrated in Fig. 3, a preventive against the occurrence of discharges between the leads, and consequent shortcircuiting of firing current, is obtained by, for example, either spacing the leads I l apart in the coherer mass (as in Fig. 2) a distance which is substantially greater than the distances between the respective leads and the detonator shell or ground, or inserting between the leads within the coherer mass (as in the case of the upper coherer plug 36 in Fig. 3) a paper sheet 42 or other insulating element.

While, in the drawings, the soldering points H and 52 are shown within the discharge plug, these points may be outside the said plug and insulated from each other and from the igniter shell by being surrounded by a rubber or similar plug.

Referring to Figs. 5 to 11:

In these figures, an alternative means of associating the igniter leads with each other is shown, such means comprising a coherer mass or masses which does not, or do not, surround the igniter leads directly, but which is or are contained in a grounded casing or casings and surrounds or surround the bared ends of insulated conductors led from the respective igniter leads.

Fig. 5 shows the application of this indirect means to a single detonator. The leads 5!), 5| of I the detonator 52 are respectively tapped by insulated conductors 53, 54, the ends 55, 55 of which are bared and surrounded by a coherer mass 51 contained in a grounded metal casing 53. Desirably, but not necessarily, the casing 58 is electrically connected to the detonator 52 by a conductor 59 and conductive sleeve 60. This ensure that the detonator casing is not at a lower potential than the casing 58 and that therefore discharge to ground will not take place through the detonator shell in preference to the casing 58.

Fig. 6 shows the application of the same means to a group of series-connected detonators 6|. The insulated conductors 53, 5'41 are in this instance led from the positive and negative wires 50, 5| of the blasting cable, and the grounded casing 58 is connected by separate conductors 59 59 and sleeves 60 to the first and last detonators of the series respectively. Otherwise the arrangement is similar to that described with reference to Fig. 5.

Fig. 7 shows another application of the said means to a similar group of series-connected detonator 62. The insulated conductors 53 54 are led from the blasting wires 58 5M and their bared ends 55 56* inserted into separate coherer masses 5'5 51 contained in separate grounded casings 58 58 This is a specially effective safeguard against short-circuiting of the firing current, e. g. between the bared ends 56, 56 (Fig. 6) of the conductors 53, 53 through the coherer mas 51.

The outside arrangement of the coherer plug shown in the preceding Figs. 5 to '7 necessitates that the construction of the plug should be robust, in order to withstand rough handling. In particular, the internal spacing of the bared ends of th conductors and the grounded casing must not be disturbed during the charging operations. A rigid easily made assembly is illustrated in Figs. 3 and 9. In thi assembly, the bared ends of the conductors 53, 54 are constituted by two strips 63, 64 of metal foil secured by an adhesive to the opposite sides of an. I shaped piece 65 of cardboard or like insulating material, which is a neat fit into the metal casing 65. Thi insulating piece 65 serves to position, and to maintain in position, the strips 63, 64 with respect to the wall of the casing 65, which is itself closed at its top and bottom ends by sealing means 61, 68 held in place by crimpings 69, H3. In the construction hown in Figs. 8 and 9 a copper wire ll is soldered at one end to the casing 66 and at its other end to a, copper ring or sleeve 12, for connection to the detonator casing (as already described with reference to Fig. 5).

Although the bared ends of the conductors in Figs. 5 to 9 have been shown as of flattened straight-edged shape, it will be understood that these ends may be of ordinary round section or the said ends and/or the casing or grounding means may be provided with discharge points as described in our co-pending patent application aforesaid.

Under practical tests, my coherer protected detonators have invariably been found capable of being fired, that is, intentionaly fired, after they have been subjected to a discharge of high voltage static electricity either through one lead only, or through both leads simultaneously. In the latter case, theoreticallyat least, a circuit permanently conductive to the firing current is created from one lead outwardly through the coherer mas to the metal casing, thence round the wall of the casing and inwardly through the coherer mass to the other lead. That such a detonator was found on test nevertheless to be capable of being fired is clearly due to the fact that the (permanently reduced) resistance of the (outward) discharge paths in the coherer mass was still high 7 enough to divert a current through the fuzehead sumciently strong to fire the same. A means of positively ensuring that the coherer detonator may be fired after simultaneous high potential discharge on both leads is shown in Figs. 10 and 7 11. In these figures, the'said means is shown applied to an outside coherer plug (as used, e. g., in the constructions shown in Figs. to 9), but it may equally well be applied to an interior coherer plug as shown in Figs. 1 to 4. The means comprises an insulating casing 13 which is closed at the top by sealing means '13 and which contains the coherer mass 14 in which the bared parts 15, 76 of the leads 11, 18 are inserted as usual. At the portion of the casing 13 directly adjacent to the bared parts 15, 16, interior conductors 79, 80 are disposed parallel to the said parts and are respectively formed integral with or conductively connected to exterior conductors BI, 82. With these separate conductors, even the permanent reduction in resistance of the discharge paths created by discharge from both igniter leads simultaneously cannot result in a short circuit of the firing current occurring between the bared parts of the leads via the casing containing the coherer mass.

A5 is the case with coherer materials in general, the part or parts of the grounded plug or mass which has or have had its or their resistance reduced by the coherer action caused by a high voltage discharge may have the resistance thereof restored to its normal high value by the plug or mass being lightly shaken.

With the present invention, an electric detonator is obtained which is substantially safe against stray currents and which, in addition, is equally applicable to series, as to parallel, blasting.

In the appended claims, the term electric leads is to be understood as including the wires of the igniter and/or the Wires of a blasting circuit, and/or conductors led from these wires.

I claim:

1. An electric igniter of a blasting detonator in a combination with means by Which it is safeguarded against accidental firing by electric shock or spark discharges produced respectively in, or in the vicinity of, the firing means of the igniter 8 by atmospheric electricity, comprising a coherer mass which is in electrical cooperation with insulated conductive branches connected with each lead of the igniter, said branches having bared parts surrounded by the coherer mass, a conductive grounded casing conductively connected to a conductive shell of the igniter, the coherer mass including a metal powder offering a substantially complete resistance to voltages of the magnitude used for firing the igniter and which, on being subjected to voltages substantially higher than said firing voltages, becomes locally conductive through coherer action along a path permitting the passage of the high voltage charge.

2. An electric igniter of a blasting detonator in combination with means by which it is safeguarded against accidental firing by electric currents or spark discharges produced respectively in, or in the vicinity of, the firing means of the igniter by atmospheric electricity including leads for the igniter common to a group of two or more detonators, insulated branches tapped from said leads and having bared parts, said bared parts being surrounded by a coherer mass which is in electrical cooperation with each lead through the insulated branches, said mass including a metal powder offering a substantially complete resistance to voltages of the magnitude used for firing the igniter and which on being subjected to voltages substantially higher than said firing voltages, becomes locally conductive through a coherer action along a path permitting the passage of the high voltage current, and a conductive casing enclosing said mass and connected electrically to the first and last of the detonators of the group when the groups are connected in series.

3. A construction as defined in claim 1, wherein the mass is common to both leads of the igniter.

4. A construction as defined in claim 2, wherein the mass is composed of finely divided aluminum flakes.

' HANS JAY ROLFES. 

